1. Field of the Invention
This invention relates to a transportation device for a camera in which film is transportated by an electric motor as the drive source.
2. Description of the Related Art
Recently, the trend to employ an electric motor as the drive source of a film transportation device has become generalized. For example, in the art of cameras, many of them have been developed with a great improvement of the manageability by motorizing the winding and rewinding of the film. Here, a problem has been encountered that when winding up the film (or when rewinding in the case of a type of camera in which all film is once wound up at the time of loading, and rewound frame by frame after each shot), the movement of the film in each frame is hardly maintained constant as the battery condition and the load on the transportation mechanism vary, with rhe result that the spacings between the successive two frames differ largely at random.
There have been previous proposals for eliminating this problem, one of which is to control each winding (or rewinding) operation in such a manner that the motor is first fully energized, then decelerated while its energization being continued by the duty drive method of speed control, and then electrically braked to insure that the movement of the film in each frame is maintained constant (as, for example, disclosed in U.S. Pat. No. 4,659,201). According to this proposal, because a proper deceleration is made to precede the braking, the one-frame movement can be maintained constant without the necessity of providing a mechanical stop means. In addition, as it leads to reduce the consumption of electrical energy, the life time of the battery can be increased.
However, in the above-identified proposal, it is in its speed control (duty drive) period that the driving force of the motor as measured in terms of the effective voltage applied thereto is constant at a lower level than when in the fully current supply. Therefore, when the actual voltage of the battery has dropped due to the exhaustion of electrical energy, or the film load has increased, it will happen that the motor is caused to stop at a time during the speed control period, giving rise to a problem that the film cannot be advanced through the full length of one frame.
Also, the device for sensing the movement of the film upon detection of a stoppage despite the film transportation being driven to take it as the end of film and for stopping the motor for film transportation even at the intermediate point during the winding operation has conventionally been incorporated into the camera. In such a camera, another problem arises that even though the drop of the battery voltage or the increase of the film load merely takes place to stop the motor in the speed control period as has been described above, this is mistaken for the completion of transportation of all the frames of film.
Another previous improved proposal is known in Japanese Laid-Open Patent Application No. Sho 60-123828. The rotation control device for the motor according to this proposal is so constructed that pulses of constant width are generated with a period depending on the speed of rotation of the motor, and the rotation of the motor is controlled by the feed back in such a manner that when the motor is rotating at a high speed, the duty ratio of the current supplied to the motor is small, and when it is rotating at a low speed, the duty ratio of the current supplied to the motor becomes large. However, because this conventional method cannot cope with the gear ratio of speed reduction of the transport transmission system which apparently varies with the number of transported frames of film (in the type of winding up the film frame by frame in each shooting, the number of exposed frames), a problem arises that the stopped position of the film is caused to change by the number of transported frames.
Still another previous improved proposal is known, for example, in Japanese Laid-Open Patent Applications Nos. Sho 58-24123, 58-24124, 58-24125. The rotation control device for the motor according to this proposal is to control the stop position of the film in such a manner that the speed of movement of the film is detected by pulses, and the amount of the film to be overrun is computed from this speed of movement by a microcomputer. However, in this previous improved proposal, the amount of overrun film varies to a large extent. When the amount of film to be overrun is large, it is necessary to stop the motor before the amount of moved film reaches a prescribed value. This led to a drawback that, if the film load increases after the stoppage of the motor, the film stops before it should be moved to the prescribed amount.